1. Field of the Invention
The present invention relates to an amplifier circuit using a class-AB CMOS operational amplifier, in particular, an amplifier circuit suitable for use as an audio amplifier circuit for a portable telephone.
2. Description of the Related Arts
An amplifier circuit using a class-AB CMOS amplifier has been widely utilized as an audio amplifier circuit, because of its various advantages as follows.
First, it consumes low amount of power since the output current, namely, the current consumed by the amplifier, is determined according to the current flowing through the loads connected to the output of the amplifier.
Second, it has small distortion in the output waveform and relatively low current flows when no signal is applied to the amplifier. In other words, the amplifier features an output waveform with a small distortion and low power consumption during idle periods.
Third, the amplifier has a relatively high output drive capacity. Fourth, the amplifier can provide an output having a relatively large voltage amplitude.
Currently, the use of portable telephones has been spreading, and various audio amplifier circuits used for the receiving circuit of such portable telephones have been explored.
There is a demand for an audio amplifier circuit used in portable telephones which is driven by a battery with a low voltage, for example, driven by a 3-V battery instead of a 5-V battery, and is low in cost.
In order to meet these demands, the inventors have tried to incorporate a receiving circuit, including an audio amplifier circuit in which class-AB CMOS operational amplifiers having the foregoing advantages are used, into a system LSI chip in which digital circuits are mainly used together with analog circuits. However, it was found that the mere direct incorporation of conventional class-AB CMOS operational amplifiers into a audio amplifier circuit for a portable telephone as it exists, makes it difficult to enlarge the output drive capacity and a voltage amplitude of the output waveform when used at a low power voltage, for example, with a 3-V voltage supply instead of a 5-V voltage supply.